Breaking of Galilean invariance in the hydrodynamic formulation of ferromagnetic thin films
Ezio Iacocca, Thomas J. Silva, Mark Hoefer
Microwave magnetodynamics in ferromagnets are often studied in the limit of small-amplitude modes that modulate a well-defined magnetic state. However, strongly nonlinear regimes, where the aforementioned limits are not applicable, have become experimentally accessible due to recent technological advances such as spin torque and the spin Hall effect. By re-interpreting the governing Landau- Lifshitz equation of motion, we derive an exact set of equations of dispersive hydrodynamic form that are amenable to analytical study even when full nonlinearity and exchange dispersion are included. The resulting equations are found to, in general, break Galilean invariance and a magnetic Mach number is obtained as a function of static and moving reference frames. The simplest class of solutions are termed uniform hydrodynamic states (UHS) and exhibit fluid-like behavior including laminar flow at subsonic speeds and the formation of a Mach cone and wave- fronts at supersonic speeds. A regime of modulational instability is also possible, where the UHS is violently unstable. The hydrodynamic interpretation opens up novel possibilities in magnetic research.
, Silva, T.
and Hoefer, M.
Breaking of Galilean invariance in the hydrodynamic formulation of ferromagnetic thin films, Physical Review Letters, [online], https://doi.org/10.1103/PhysRevLett.118.017203, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=921162
(Accessed November 30, 2023)